Light in a mid-infrared region (e.g., where the wavelength is on the order of 5 μm to 30 μm) is an important wavelength region in the field of spectroscopic measurement, for example. Attention has been focusing on quantum cascade laser (QCL) elements as high-performance semiconductor light sources in such a wavelength region (see, for example, Japanese Patent Application Laid-Open Nos. H08-279647, 2008-177366, 2008-60396, and H10-4242).
The quantum cascade laser element is a laser element of monopolar type which utilizes a level structure caused by subbands formed in a semiconductor quantum well structure, so as to generate light according to electronic transitions between the subbands. The quantum cascade laser element achieves highly efficient, high output operations by cascade-connecting multiple stages of quantum well light-emitting layers, each of which is constituted by a quantum well structure and serves as an active region. The cascade connection of quantum well light-emitting layers is attained by using electron injection layers for injecting electrons to upper emission levels and stacking the quantum well light-emitting layers and injection layers alternately.